how to bisect_plane with use_fill = True in Blender 2.79 using python?
Boolean Modifier

Can use the clear_inner or clear_outer property of the bisect plane operator, and run twice, grid fill the returned cut(s).

However it may be much simpler to script this approach




add a plane at cutting coordinates
scale the plane such that it completely encapsulates mesh to be cut (sphere in example image)
give the plane thickness with solidify modifier, once again making sure it completely encaps cuttee mesh.
add a boolean modifier (difference)  to the object to be cut, using plane as target.
apply the modifier and save result.
redo with the plane normal flipped (other half).
the resultant filled face will be an ngon(s).